DESCRIPTION: The majority of tumor-targeted therapies currently used in the clinic are comprised of antibodies or small molecule inhibitors aimed at blocking growth signaling. However, signal-blocking therapies have been ineffective in a majority of cases due to mechanisms that sustain signaling in the face of targeted treatment, highlighting a need for alternative strategies that do not rely on signal-modulation. In our previously funded project, we developed self-assembling protein-corrole constructs that circumvent the need to modulate signaling by using tumor cell surface biomarkers as portals for the targeted entry of corrole molecules. Sulfonated corroles are water soluble, macrocyclic compounds that may be metallated, spontaneously assemble with proteins, and can be cytotoxic as well as bear various photophysical properties for both imaging and diagnostics. We have shown that sulfonated corroles are membrane-impermeable yet require cytoplasmic entry to elicit cytotoxicity while remaining excluded from the nucleus. Our targeted cell penetration protein, HerPBK10, enables corrole uptake into human epidermal growth factor receptor subunit- 2 positive (HER2+) tumors via specific interaction with the HER2 dimerization partner, HER3, which is particularly represented on these cells. Receptor-binding triggers rapid endocytosis followed by endosomal escape via a membrane-lytic domain on HerPBK10, enabling corrole entry into the cytoplasm. The tumor- homing nanoparticle, HerGa, formed by assembly of HerPBK10 and a gallium metallated corrole (S2Ga or Ga- corrole), can target and ablate HER2+ tumors in mice at >10x lower dose compared to conventional chemotherapy while sparing heart and liver tissue, and with no detectable immunogenicity. Studies in recent years have now discovered that elevated cell surface levels of HER3 is associated with resistance to a number of signal-blocking breast cancer treatments, including inhibitors of EGF-R or HER1 (lapatinib), HER2 (lapatinib, trastuzumab, T-DM1), HER2-3 (pertuzumab), and combination therapy. Moreover, HER3 elevation has been identified on metastatic breast tumors, including those that spread to the brain, and on untarget-able tumors such as triple-negative breast cancer (TNBC), including TNBC with acquired resistance to EGF-R inhibition. The HER3 specificity of HerPBK10 predicts that tumor cells resisting these signal-blocking treatments are prime targets for HerPBK10-directed nanobiologics. The present study will explore this on models of resistant and metastatic breast cancer, especially those that metastasize to the brain. A regimen of using EGFR and HER2 inhibitors as adjuvants to sensitize tumors to corrole nanobiologics will be evaluated. As the median survival of patients with metastatic breast cancer is 3 years, and patients with breast cancer metastases to the brain on average survive less than one year, improved alternatives are urgently needed.